In recent years, one of the key issues in the aircraft industry is to improve the fuel efficiency of aircrafts from the viewpoint of reduction of aircraft operation costs as well as environmental conservation. Accordingly, reduction in power loss of planetary gear systems for use in aircraft engines is also required.
Conventionally, in such a planetary gear system, a double helical gear which includes a pair of helical gears aligned in the axial direction is used as a planet gear, aiming at compensating for axial loads (see Patent Literature 1, for example). The double helical gear is rotatably supported by a fixed support shaft via a bearing so that stable support can be realized. The bearing includes a pair of rolling element rows that are spaced apart from each other in the axial direction. Each rolling element row includes multiple rolling elements that are arranged in an annular manner around the fixed support shaft.